In electric power industry, demand is an average value of power during a preset cycle, such as fifteen minutes, and maximum demand is a greatest value of the demand during an electric charge accounting period, such as a month. The maximum demand usually is predetermined by a consumer. Electric charges of the consumer are calculated according to the maximum demand predetermined by the consumer, if the predetermined maximum demand is too high, more basic electric charges may be applied, if the predetermined maximum demand is too low, more extra electric charges may be applied.